Furnace muffle



J. H. BECK FURNACE MUFFLE Nov. 12, 1968 Filed Oct. 5, 1966 FIG.2

INVENTOR a. HOWARD BECK FIG.4

ATTORNEYS United States Patent 3,410,544 FURNACE MUFFLE Jacob Howard Beck, Waban, Mass., assignor to BTU Engineering Corporation, Waltham, Mass., :1 corporation of Massachusetts Filed Oct. 3, 1966, Ser. No. 583,842 4 Claims. (Cl. 263-41) ABSTRACT OF THE DISCLOSURE A fluid cooled furnace muffle having a cooling chamber formed with spaced inner and outer metallic jacket sections, each jacket section having a pair of adjacent longitudinal edges. Adjacent edges of both pairs are arranged to be in contact and are secured by means of a single longitudinal weld which also seals the cooling chamber. The muflle may be strengthened and the jacket sections rigidly spaced by means of indentations in the outer section welded to the inner section.

The present invention relates in general to industrial furnaces and more particularly to a new and improved construction for a furnace muflie having a chamber for containing a fluid heat-transfer medium.

In a substantial number of continuous heat processing systems, as for example, in the art of manufacturing semiconductor products, the work pieces are heated while transported on a conveyor through a relatively long tubular muflle. It is common in certain processes to provide a muffle section which is jacketed for the flow of cooling water or some other fluid to achieve a rapid temperature drop in the terminal stage of the process.

All fluid jacketed systems present inevitable problems; perhaps the most significant being the possibility of internal fluid leaks. In the case of furnace muflles, such leaks, however slight, are especially troublesome since fluid vapors in the muffle interior effectively alter the gaseous atmosphere under which the particular heating and cooling cycle is being performed. As a specific example in the high temperature processing of semiconductor materials, the presence of even trace amounts of water vapor may result in the total loss of relatively expensive components.

Fluid leaks may of course be repaired whenever found, usually by welding over the defect. On the other hand, if one considers the fact that furnace muffles are inherently long hollow tubes of relatively small internal cross-sectional area, it may be observed first that the detection of internal fluid leaks is at best exceedingly difficult; and; most often such leaks remain undiscovered until defective products point to a system failure. Further, the discovery of a fluid leak somewhere in the mid-section of a furnace muffle which may be ten feet long and which may have a maximum cross-sectional dimension of six inches, or even less, poses an extraordinarily difllcult repair problem; indeed, it may be necessary to disassemble the furnace and the muflle merely to repair a pinhole leak. The process time lost thereby and the attendant high cost of such repair is self-evident.

A conveyor type furnace muflfle usually is formed from sheet metal stock, as for example the nickel alloy sold under the trademark Inconel, which is adapted to withstand not only the high temperatures attained during the process cycle but also the various chemical vapors and gases which serve as controlled atmospheres therein. This sheet metal is ordinarily folded in a bending brake, in one or more sections, to the desired mutlle cross-section which may be rectangular, pentagonal, or the like, and the various sections are assembled by a technique appropriate for the particular alloy.

3,410,544 Patented Nov. 12, 1968 Where a fluid jacketed muffle is required, it is accepted practice to pre-form complete inner and outer mufile sections, assemble one within the other on suitable spacers, and to seal the open ends to close the fluid jacket. One or more inlet and outlet ports and valves are provided to permit circulation of cooling fluid at the desired rate. With the customary structure just described, external leaks in the water jacket are not diflicult to locate and repair by further welding. Internal leaks through the welds in the inner muflle jacket, which leaks result in fluid passage into the process volume, cannot be detected by ordinary visual inspection during normal furnace operation and virtually require disassembly of the muffle for discovery.

It is accordingly a primary object of the present invention to provide a fluid jacketed mufile construction which not only minimizes the possibility of fluid leakage to an exceptional degree but wherein leakage, if such occurs, is inherently confined to a limited region available for immediate visual inspection and uncomplicated, low-cost repair.

A further object of the present invention is to provide a fluid jacketed muffle construction which substantially precludes the possibility of internal fluid leakage.

Another object of this invention is to provide a fluid jacketed muffle wherein a single lengthwise weld is utilized to join together and seal a pair of muflle sections, and wherein the placement of this weld is on the outermost surface of the muflle, available for continuous inspection and ready repair.

Still another object of the present invention is to provide a mufile which combines the foregoing features with high rigidity, minimum vibration, and which substantially eliminates the possibility that a muflle wall will collapse or burst under the inadvertent application of high fluid pressure.

Broadly speaking, the fluid jacketed muflle of this invention is formed of a pair of unitary, folded sheet metal sections arranged one within the other so that all of the lengthwise seams may be effectively sealed by a single longitudinal weld located on an exterior surface of the muflle. End plates may be welded to the outer ends of the muffle to define the fluid cavity, the welded edges between the end plates and the muflle sections being located as that they too are on the exterior visible surface for convenient inspection and repair as required.

The construction, operation and attendant advantages of the novel fluid jacketed muflle of this invention will become readily apparent from the following detailed description when taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a fragmentary perspective view of a relatively short axial length of an elongated fluid jacketed muffle which embodies the principles of the present invention, the muflie portion being shown in association with a diagrammatic representation of a process conveyor belt;

FIG. 2 is a cross-sectional view of the novel fluid jacketed muffle of this invention taken along the plane 22 of FIG. 1;

FIG. 3 is an enlarged fragmentary cross-sectional view of one of the indentations or dimples in the outer muffle wall prior to welding; and

FIG. 4 is a fragmentary enlarged cross-sectional view of the novel fluid jacketed muffle taken along the plane 44 of FIG. 1.

With reference now to the drawings and more particularly to FIG. 1 thereof, there is shown one end of a fluid jacketed muffle which may be employed in a typical process furnace which utilizes a mesh-type flexible conveyor belt 10, driven over pulley 12 by conventional power means not shown. Since the novelty of the present invention resides in the design and construction of the fluid jacketed mufiie itself, all other details of the furnace structure, including the longitudinal heaters, temperature barriers, thermal insulation, gas inlets and outlets and temperature sensing and control elements, have been omitted from the drawing. The discussion which follows will be limited essentially to a description of the muflle configuration and its operation and particular utility. It will be understood that the novel muffie may be incorporated into conventional furnaces or even substituted for prior muflle designs in older furnaces.

The novel fluid jacketed muflie 14 is shown as formed with an internal muffle section 16 having a pentagonal cross-section with a flat internal base 18 (see FIG. 2) which supports the movable conveyor belt 10. The internal peaked, attic-like area is especially useful for the axial flow of lighter-than-air gases which may be used as controlled atmospheres in certain processes.

With further reference to FIGS. 1, 2, and 3, the internal mufiie section 16 is wholly enveloped by an external muffle section 22 of similar cross-section, as shown, but larger in area than the cross-section of the internal mufile section 16. Both the inner muflle section 16 and outer muflie section 22 are shown as formed from individual, unitary metal sheets folded in a bending brake to the respective pentagonal shapes shown and formed with the upstanding complementary, lengthwise edge portions 20a and 2015 on the inner section 16, and 24a and 24b on the outer section 22. As shown most distinctly in FIG. 4, all four of these edges are arranged to lie neatly and flatly one against the other and to provide a uniform exposed upper edge surface which is sealed by a single lengthwise weld 26, as clearly illustrated in FIGS. 1, 2 and 3.

The welding technique utilized to seal the complementary edges with weld 26 is a function of the sheet stock utilized in the formation of the inner and outer muffle sections. Where Inconel, or a like stainless steel, is employed, an excellent, substantially flawless weld may be made by using the Heliarc technique. With this weld ing process, an electric arc is drawn from the upper exposed surface of the edges 20a, 20b, 24a, and 24b to an inert rod and the Inconel material is melted and fused together under an appropriate inert atmosphere to form the continuous metal sealing bead 26. Under the circumstances all of the metal, including the weld 26 shown in cross-section in FIG. 4, is derived entirely from the original Inconel stock, and consumable welding rods are not used. While this is a convenient process for providing a reliable, durable weld, others may prefer the use of consumable metal wire welding, brazing (if the working temperature of the muflie permits), or any other appropriate leak-free sealing technique. In the context of this patent application the term weld will be deemed to embrace any of the useful sealing techniques.

The welding together of the inner and outer mufl'le sections 16 and 22 respectively by weld 26 will thus be seen to provide an axially symmetric, longitudinal fluid chamber 30 which, when sealed at the muflle ends, will permit the confinement of a cooling fluid, such as water 32.

With specific reference to FIG. 1, the fluid chamber 30 of the muffle is sealed by a complementary pair of end plates, one of which 28a is illustrated, the other of which is at the opposite end of the muflle and hence is not shown. The end plate 28a fits over the cross-section of chamber 30 and is welded appropriately to the edges of the inner and outer muflie sections 16 and 22. To provide for fluid circulation, a pair of circular openings 34a and 34b are formed in the outer muflie section 22, which openings communicate with the interior of the fluid jacket, namely, chamber 30. A pair of complementary inlet and outlet pipes 36a and 36b, respectively, are welded into outer muffle section 22 at the ports 34a and 34b as a result of which fluid pumped into pipe 36a will circulate in the chamber 30 and flow outward through pipe 36b. Inlet and outlet fluid control valves are not shown but may be connected in pipes 36a and 36b in the customary manner. The relative sizes and positions of the ports 34a and 34b will be a matter of design choice, depending, for example, on the amount of fluid which must be circulated to achieve the desired cooling effect; however, it is preferable that the ports 34a and 34b and their respective inlet and outlet pipes be arranged for ready visual observation to facilitate the detection of any leakage in these welds, although it should be observed that such leakage, while undesirable, will not contaminate the product being conveyed through the mufl le.

As a particular feature of the present invention, reference is made to FIGS. 14 inclusive, all of which show a plurality of inwardly pressed indentations or dimples in the outer muflie section 22, the depth of each which is, as best shown in FIGS. 2 and 4, just suflicient to position and support the inner muflie section 16. These dimples are provided in the sidewalls and the base of muflle section 22 and, as best illustrated in FIG. 3, are each formed with a central punched hole 40.

As most clearly shown in FIGS. 1 and 4, a weld 42 joins the peripheral edge of metal surrounding each opening 40 in muflie section 22 to the outer surface of the corresponding area in muflle section 16. It is evident that this weld requires the addition of metal and accordingly a consumable and compatible welding rod is used for this process. The weld 42 is carefully inspected to insure complete closure of the opening 40 in each instance thus precluding leakage of fluid from the chamber 30 to the outside of the mufiie. Notwithstanding, should a leak develop in any weld 42, fluid would flow outwardly of the muflie and no process contamination would be involved. Also, such leakage would be readily ascertainable by visual inspection of the muflle exterior.

The advantages of the dimpled construction and this multi-point attachment of the outer muffle section 22 to the inner muffle section 16 will now become apparent. The dimples provide an extremely convenient means for centering and positioning the inner muffle section 16 in symmetrical manner with the desired spacing to provide a uniform cross-section for the fluid chamber 30. In addition, by virtue of the welds 42, a rigidized structure is created which will prevent undesired vibration or rattling of the inner muffle section which might occur due to pulsations of pressure in the fluid medium 32 flowing through the chamber 30.

Finally and significantly, the attachment of outer to inner muffle sections in this manner markedly strengthens both inner and outer sections from the standpoint of resistance to the application of high fluid pressures within the chamber 30. More specifically, in any fluid flow system as that shown herein utilizing pipes 36a and 36b, the fluid pressure within chamber 30 is considerably less than the static pressure of the fluid source (not shown) connected to pipe 36a, the lower pressure being due to the relatively small pressure differential necessary to circulate fluid through to the outlet 3612. However, if inadvertently the outlet valve (not shown) in the pipe 36b is turned off while pipe 36a is connected to the fluid source, or if the outlet pip 36!; becomes clogged in some manner, the full pressure of the fluid source will build up as a static pressure in the chamber 30. Under ordinary conditions the flat sidewalls of the inner muflle section 16 might buckle inwardly while the corresponding sidewalls of the outer muffle section 22 could buckle outwardly or even burst. In either case the damage would be virtually irreparable and mutfle replacement would be required. Through the use of the dimples 38 each welded as at 42, a rigid pressure resistant structure is achieved without sacrifice of the primary advantages of the novel muflie configuration, which precludes fluid leakage into the mufile interior.

Summarizing, an examination of the novel, improved mufile reveals that the problem of concealed leakage or seepage of fluid within a jacketed mufile has been eliminated, while at the same time manufacturing costs have been substantially reduced through the sharp reduction in the number of welds required. The novel mufile construction may be used on any size-scale and virtually for any length without limitation other than the availability of appropriate sheet metal bending machinery.

While a simplified pentagonal cross-section has been illustrated in the drawing, it should be clear that the cross-sectional shape is not a limitation to the scope of this invention. Further, while the pentagonal shape shown is most conveniently achieved by stacking the longitudinal edges 20a, 20b, 24a, and 2412 at the top of the mufile, it would be equally possible and practicable to form the exposed edges and join them at some other point along the peripheral edge of the device.

It is to be understood, therefore, that this invention is not limited in its application to the precise details of construction and arrangement of parts described and illustrated in the foregoing specification and that the invention is limited solely by the spirit and scope of the appended claims.

What is claimed is: 1. A mutfle for a conveyor furnace comprising: an elongated, unitary inner mufile section formed with a pair of external adjacent longitudinal edge portions;

an elongated, unitary outer rnufiie section, enclosing but spaced from said inner section and formed with a pair of external longitudinal edge portions disposed in contact with and adjacent to respective edge portions of said inner section;

a unitary external elongated weld arranged to seal together said two pairs of adjacent edge portions;

means for closing opposite ends of said mufile in the region between said inner and outer mufile sections, thereby to define a hollow chamber lengthwise of said mufiie between said sections; and

means for circulating the fluid through said chamber.

2. The furnace muffie of claim 1 wherein said outer muffle section is formed with a plurality of inwardly projecting indentations, each of said indentations having a substantially central opening therein, said outer mufile section being welded to said inner muffle section along the periphery of each of said openings; whereby the joining of said inner and outer muflle sections by said welds in said indentations provide increased structural rigidity to said mufile.

3. The furnace muflie in accordance with claim 1 wherein said external elongated weld comprises fused portions of said longitudinal edges.

4. The furnace mufile in accordance with claim 1 wherein said ends of said muffle are sealed by end plates welded to the respective ends of both said inner and outer muffie sections; said end plates being formed witlrcentral openings therein substantially corresponding to the internal cross-section configuration of said inner mufiie section.

References Cited UNITED STATES PATENTS 833,640 10/1906 Smallwood 26339 X 1,782,234 11/1930 Hofmann 29-455 2,391,997 1/ 1946 Noble 29455 2,602,653 7/1952 Cope 263--44 X 3,058,861 10/1962 Rutter 29455 3,160,403 12/1964 Diman et al. 2638 JOHN J. CAMBY, Acting Primary Examiner. 

